Asking for help with oven build

[SClearman]

First I have to admit, I'm not heat treating knives. I've 'built' several knives for myself and friends from pre-made blades and blanks. What I need the oven for is to heat treat my main project, suppressors. Everyone who has decided to or wanted to build an oven knows the commercial options are expensive. I need to be able to heat treat a completed suppressor after welding together. I'm using 17-4ss for all the parts, and everything is autogenous welded together. The heat treat is fairly simple, heat to 900*F and hold for one hour, then cool in room air to ambiant. No quench.

I'm already building the oven. I have 2300* bricks from lynn. The oven will be basically square, each wall, foor and top will be 13.5"x13.5", with a front loading door. I intend to notch each joint and overlap, so my internal chamber should be 11.5"x11.5"x11.5". I'm going to call it one cubic foot chamber. The ouside will be insulated with 1" Koawool. It will sit on a metal framework and the outside skin will be thin sheet metal.

I started with a smaller Paragon kiln, 6x6x6". I build a PID controler system for it. I'll reuse that and add another SSR so it can use the 220/230V I have available in the shop.

That's the plan, now what I need help with. Specifically the heating elements. 2 questions probably related:
1: am i correct that the length of the heater coil is determined by how much amperage I wish to pull?
1a: will any length (short of what the max availible amperage is available) heat up fully? What I mean is the Kanthal wire heats to "X" degrees. does it heat to that not matter the amperage? for example, if i have 50amps available, and I measure out the correct length of wire to use all 50 amps (hyhpothetical I know i have to use less) the wire will heat up to a specific temp, will a shorter piece of wire heat to the same temp and pull less amps? and if so does that mean that any length of coil, less than the maximum, will heat to the same temp but because of how much element there is a shorter coil will heat the chamber slower?

That was more than 2 related questions.

2: how many coil channels should i try for in the oven?

Maybe very basic but I would definatly appreciate some guidence.

 

[Stacy E. Apelt - Bladesmith]

Welcome to Shop Talk.

Your 900°F HT is correct.
(For those here who HT knife steel, the HT for 17-4, AKA H900, is a precipitation hardening steel, and does not need the higher temperatures.)

You might want to look in the places that sell stuff for a used dental or jewelers oven (AKA burnout kiln). They can often be picked up for around $100. That will be more than sufficient for your 900°F process.

Building a knife type HT oven will cost more and may or may not work... depending on your skills and understanding of te electronics.
If you want to make your own, use the Custom Search Engine in the Stickys. Search "HT oven build". There are many threads with great info and tutorials.

Best of luck.

 

[SClearman]

Thanks. Ive looked and looked and the size I need is the problem. Finished suppressors are up to 9" length and building it large enough to batch treat them is the goal. the H900 is the heat treat I'm using but there are others such as H1150 which is a heat to 1150*F and hold for an hour. Ironically the higher temp treat produces a softer finished product.
I think I've got the basics of the build down. Really needing some clarification on the elememts.

 

[SClearman]

I'm seeing wattages all over the place. What is a 'normal' wattage/cubic foot to shoot for?

 

[Stacy E. Apelt - Bladesmith]

For your purpose, I would stay with H900.
You don't need a huge wattage oven for 900°F. For that range, 1500W will be enough. If you make a larger chamber, 2000W will be fine.


I spotted a big burnout oven on the Bay for $10. Needs some wires re-connected. Unfortunately, it is in Salt lake City.

I think for your temps, quartz heating rods will work just as well as coils. Hopefully, Taz or one of the oven build guys will chime in.

 

[Taz]

I would look for a 14" x 6" diameter kiln on FB marketplace. Plenty of room and heat for what you want, just rig the PID to it.

If you want to build, I would go with a longer chamber, narrower and shorter. It will be a lot more useful for knives, easier to do coils in and take less time/energy to heat up! Lots of guys do 14" or 18" long kiln, 4" or 5" height and width. I do larger Shechita, so my oven is 27" deep, 7" wide and 6.5" tall. I used the electric wiring from Red Beard and Jarod Todd's method of fused quartz tubing to support the element coils going across the top of the chamber in the oven. I found a premade kiln element that was perfect for my oven. Coil over tube let's you go with thicker Kanthal and more of it, as well as being a more even distribution throughout the oven instead of stuffing the coils into the walls.

 

[Stacy E. Apelt - Bladesmith]

Excellent idea, Taz.
A pottery or glass kiln is often very cheap used. I checked the OP's area and there are several within a reasonable drive. Just change the control to a PID and it can be set to run at any temp up to over 2000°.

 

[Taz]

Evenheat GT 14-6 with PID.

 

[Stacy E. Apelt - Bladesmith]

Funny coincidence... I have the same kiln and used a plastic tool box from Harbor Freight to mount the PID controller. I cut a hole in the top and put a small salt tube in the kiln. I use it for marquenching and for niter bluing salts. I set the kiln to MAX and let the PID control the temp.

I recently turned down two free huge paragon kilns that were 38" deep and 30" wide. While I could have done HT for swords in them, they required 60 amp 220 volt lines to run them.

 

[SClearman]

Thank you gentlemen for the suggestions. Again, this is for suppressors not knives, at least not at this point. I have looked at some of the kilns and have decided to build. I'm already in the building process as a matter of fact.

I still haven't found all the answers I need.

 

[Stacy E. Apelt - Bladesmith]

Use the custom search engine to search "HT Oven Build". You can add Red Beard or Jarod Todd to that search for their build threads and info.

 

[Taz]

It's a pain to run that many elements to get a cube to heat up evenly. A longer, narrower and shorter oven would work better for you. You could copy red beards oven completely and do what you need to do more efficiently than the cube you are building and not have to figure anything out since he goes over all of the elements, wiring, etc.

There are several build threads that talk about coil length, but it's not that simple. How many amps you have, wire gauge (ie resistance), how much length you have for the coils, etc all comes in to play.

For my oven, I found a premade coil that pulled 16 amps at 240v, 3840 watts and when stretched out properly, gave me enough coils length to fit my oven very well.

Most kilns are square or octagon because they need to be able to fit several pieces or larger size pieces in them, but are very inefficient and take a very long time to get to temperature. You could also do a vertical oven and hang the silencers straight down, too.